Avoid a stack-trace when the ratelimit is hit

core/admin/mailu/internal/nginx.py

@@ -23,6 +23,7 @@ STATUSES = {
     "ratelimit": ("Temporary authentication failure (rate-limit)", {
         "imap": "LIMIT",
         "smtp": "451 4.3.2",
+        "submission": "451 4.3.2",
         "pop3": "-ERR [LOGIN-DELAY] Retry later"
     }),
 }